Delphi error "E2064 Left side cannot be assigned to" 64 bit - delphi

Question

I am trying to compile the Magenta packet capturing units to 64 bit.
https://www.magsys.co.uk/delphi/magmonsock.asp
When I compile I get the dreaded Left side cannot be assigned to on the following line of code
LongWord(bp) := LongWord(bp) + BPF_WORDALIGN(caplen + hdrlen);
This is because the LongWord cast is different byte length in 64 right? Can anyone help properly fix the line so it compiles happily in 32 and 64 bit versions? bp is declared as a pointer. caplen and hdrlen are declared as integers.
The BPF_WORDALIGN function is
function BPF_WORDALIGN(X:LongWord) : LongWord;
begin
result := (((X)+(BPF_ALIGNMENT-1))and not(BPF_ALIGNMENT-1));
end;
Thanks for any help to get this working.
Here is the full procedure with the faulty line if that helps;
function pcap_read( p:PPcap;cnt:integer;CallBack:Tpcap_handler;User:pointer)
: integer;
var cc : Longword;//Counter ?
n : integer;
bp,ep: pointer; //Begin and End Point ?
//bhp : Pbpf_hdr;//pointer to BPF header struct - removed by Lars Peter
hdrlen, //Length of Header
caplen: integer;//Length of captured
begin
if NOT LoadPacketDll then
begin
p.errbuf := 'Cannot load packet.dll';
result:=-1;
exit;
end;
cc := p.cc;
n := 0;
if p.cc = 0 then
begin
// *Capture the Packets*
if PacketReceivePacket(p.adapter,p.packet,TRUE)=false then
begin
// ERROR!
p.errbuf :='Read Error: PacketRecievePacket failed';
result:=-1;
exit;
end;
cc := p.packet.ulBytesReceived;
bp := p.buffer;
end else bp := p.bp;
// Loop through each packet.
ep := ptr(longword(bp)+cc); //move end pointer
while (longword(bp) < longword(ep) ) do
begin
caplen := Pbpf_hdr(bp).bh_caplen;
hdrlen := Pbpf_hdr(bp).bh_hdrlen;
// XXX A bpf_hdr matches apcap_pkthdr.
callback(user,
Ppcap_pkthdr(bp),
ptr(longword(bp)+longword(HdrLen)));
LongWord(bp) := LongWord(bp) + BPF_WORDALIGN(caplen + hdrlen);
inc(n);
if (n >= cnt)and(cnt>0) then
begin
p.bp := bp;
p.cc := longword(ep)-longword(bp);
result := n;
exit;
end;
end;
p.cc := 0;
result:=n;
end;